1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a method of maintaining synchronization by detecting a burst sequence in wireless communication systems.
2. Description of the Related Art
Wireless communication systems principally involve voice and data communications between a mobile station and a base station. Most of the wireless communication systems operate on a set of standards that regulates the transmission frequencies, protocols, and other communication specifications. An example of the telecommunication standard is Global System for Mobile communication (GSM). FIG. 1 illustrates the layout of a conventional GSM network. A mobile station (MS) 100 is carried by a mobile subscriber. A base station subsystem 118 includes at least one base station (BTS) 102 and a base station controller (BSC) 104. The base station subsystem 118 controls the radio link between the MS 100 and the BTS 102. The central of a network subsystem 120 comprises a mobile services switching centre (MSC) 106. The MSC 106 performs the switching of calls between the MS 100 and other fixed or mobile network users, as well as management of mobile services. The MSC 106 connects with a gateway MSC (GMSC) 108, an equipment identity register (EIR) 110, an authentication centre (AUC) 112, a home location register (HLR) 114, and a visitor location register (VLR) 116.
In GSM system, each subscriber does not employ a communication channel alone, but shares the communication channel with up to seven other users. GSM system is an application of Time Division Multiple Access (TDMA). In a TDMA system, the received signal is not a continuous stream, but consists of timeslots separated by guard periods.
When a subscriber switches on a mobile station 100 to communicate with a base station 102, the mobile station 100 must establish time synchronization with the base station 102 to properly receive information. As shown in FIG. 2, the mobile station 100 has an internal frequency reference, typically a crystal oscillator 216. The frequency of the crystal oscillator 216 varies with temperature, time, and environmental conditions. The crystal oscillator 216 provides a reference frequency to a radio frequency (RF) phase lock loops (PLL) 220 and an intermediate frequency (IF) PLL 218. The RF PLL 220 generates a local oscillator frequency to down-convert the received RF signal. The PLL local oscillator frequency must be precisely matched the frequency of the received signal to accurately modulate information carried by the signal. In order to maintain the synchronization between the mobile station 100 and the rest of the network, the local oscillator frequency is periodically adjusted by a burst sequence in a Frequency Connect Channel (FCCH). The FCCH burst sequence is a sinusoid (a section of sine wave) having a frequency equals to a quarter (¼) of the transmission bit rate. For example, the transmission bit rate of the GSM system is 270.8 kilo-bits per second (Kb/s), so the burst frequency is 67.7 Kilohertz (kHz). The FCCH burst sequence will last for 577 micro second (μs), as each timeslot in the GSM system is 577 μs.
Conventional methods for burst frequency identification are generally divided into three categories: frequency, phase, and magnitude. Frequency detection methods are widely used because frequency is usually immune to noise and distortion. The burst frequency is identified by observing an acute peak in the frequency domain. Phase detection methods identify the burst frequency by storing and examining the phase of each sample in a received signal. If the phase increases linearly for a period of time, the received signal is assumed to be a sinusoid and the rate of change of phase is the frequency of the sinusoid. Amplitude detection methods are usually not applicable because amplitude is too sensitive to noise and distortion induced by environmental factors. However, the frequency detection methods need a spectrum transformation such as Fast Fourier Transform (FFT) to convert frequency into time, and the spectrum transformation is usually very time-consuming. Thus, the frequency detection method is not suitable for real-time applications such as FCCH burst search in the GSM system. The calculations involved in the phase detecting method as well as detecting a line with uncertain slope are relatively complex.
U.S. Pat. No. 6,393,071 describes a method for identifying a FCCH burst frequency by determining the mean and variance of zero crossings of the sampled data. The FCCH burst frequency is identified if a calculated indicator becomes greater than a predetermined threshold. The start of the FCCH burst sequence is determined by the minimum variance of the burst frequency. The invention disclosed in this US patent identifies the FCCH burst sequence based on observing the periods between consecutive zero crossings, as the periods are expected to be the same if the sampled data is a sinusoidal with constant frequency.
The present invention provides a delta-phase detection method for real-time identifying the frequency of a burst sequence in a received signal, as well as locating the end of the burst sequence. In comparison with prior art, the method disclosed in the present invention only requires simple calculations to identify the frequency of the burst sequence.
The delta-phase detection method is valid for identifying burst sequences of any length in a wide range of frequency.